The invention relates to an optical amplifier comprising an active fiber, a pump unit spaced from the active fiber and adapted to give a nominal, continuous pump power in an operational state, and a pump fiber adapted to transfer pump power from the pump unit to the active fiber.
The invention moreover relates to a method of preventing emission of optical power exceeding a prescribed safety limit upon interruption of an optical fiber which transfers pump power from a pump unit to an active fiber.
Optical fiber amplifiers for amplifying optical signals typically consist of a length of active fiber, which may e.g. be an erbium-doped fiber, and a unit for generating pump power, e.g. a pump laser. When the active fiber is pumped with a strong optical signal (the pump signal) having a wavelength range different from that of the signal to be amplified, and a communications signal is launched into the amplifier, a signal coherent with the signal on the input will occur on the output of the active fiber. The gain is determined i.e. by the power of the pump signal.
The active fiber may be arranged at a considerable distance (e.g. 10-50 km) from the pump laser, in which case the amplifiers are referred to as remote-pumped amplifiers. With e.g. remote-pumped preamplifiers, also called RILP (Remote In-Line Preamplifier), the active fiber is thus spaced from the actual receiver of the optical signals, and it is pumped from the receiver. This takes place via an optical fiber, typically, but not necessarily, the same fiber as transmits the communications signals from the active fiber to the receiver.
The light transmitted in such fibers, in the form of communications signals or pump power, is typically harmful to the human eye. Therefore, because of situations with access to fiber ends or non-connected connectors, it is prescribed by various standards how much optical power may be transmitted from an open fiber end in these situations. These situations may e.g. occur in case of repair, maintenance and testing of systems, or when a fiber has broken, or a connector is disassembled. It is the temporal mean power of the light that is harmful to the eye.
To achieve the desired function of a remote-pumped amplifier, it is necessary to emit levels of pump power in the fiber from the pump laser which significantly exceed the mentioned safety limits. To comply with the safety standards, it is therefore necessary to reduce the pump power in the event that the fiber transmitting the pump power is interrupted between the pump laser and the active fiber.
Further various communications equipment standards prescribe that the equipment must be capable of automatically resuming normal operation when the transmission path has been re-established after a break and transmission signals are transmitted again. For remote-pumped amplifiers, such as e.g. RILP, this requirement, however, is not easy to satisfy, as the reduced pump power results in a considerable reduction in the gain of the active fiber. Therefore, the communications signals arriving at the receiver after the re-establishment of the transmission path, will frequently be below the sensitivity limit of the receiver because of the reduced pump power.
This problem has previously been solved e.g. by using an additional fiber from the receiver to the active fiber. This fiber, in combination with the transmission fiber, is used for passing a control signal from the receiver to the active fiber and back to the receiver. When the control signal is present, there is no break on the fiber and consequently no access to the strong optical pump power, and the pump laser can therefore pump with full power. When, on the other hand, the control signal is absent, this indicates a break on the fiber, involving the risk that the optical power hits an eye, and the pump power is therefore reduced to a safe level until the control signal is present again.
Although this solution is technically adequate, it is vitiated by the serious drawback that it requires an additional fiber typically of a length of 10-50 km. Moreover, a detector or a coupler capable of returning the control signal to the receiver must necessarily be provided at the active fiber.
Systems which are able to reduce the optical output power from a fiber amplifier in case of a broken fiber are also known. These systems do not involve remote-pumped amplifiers and, therefore, they only reduce power of the communications signals because the pump power never leaves the fiber amplifier itself.
Such a system is described in DE 42 22 270 in which the pump power to the active fiber is reduced if an alarm signal is received from the receiver in the other end of a transmission fiber, said alarm signal indicating that the communications signals are not received, e.g. because of a broken fiber. However, this can only be done if there is an extra fiber or another transmission channel for transfer of the alarm signal and, therefore, this system also has the above-mentioned drawback. Further, the system is not suitable for reducing pump power, unless a special detector unit as above is provided at the active fiber for generation of an alarm signal.
A similar system is known from U.S. Pat. No. 5,428,471 in which two parallel fibers are used for transmission in respective directions. When a fiber amplifier in one direction detects an absent input signal a message is sent via the opposite fiber back to the previous fiber amplifier to reduce or shut down its optical power level. Therefore, also this system has the above-mentioned drawbacks.
The invention provides an optical amplifier of the stated type which, in case of a pump fiber break, is capable of complying with the standards of how much light may be transmitted on the fiber, and which is simultaneously capable of returning to full pump power when the fiber connection has been re-established. This may take place by using the existing fiber or fibers, which means that no additional fiber is required exclusively for this purpose, and that an additional detector or coupler at the active fiber is obviated.
This is achieved according to the invention in that, in a safety state, the pump unit is moreover adapted to give a pulsed pump power whose mean power is lower than a prescribed safety limit.
Pulsing of the pump power ensures that its mean power can be kept so low in the safety state that the emitted light is unharmful to the human eye, while the instantaneous power of the pulses is sufficiently high for the active fiber to respond on reception of these pulses and to inform the pump unitxe2x80x94via the pump fiber or optionally another existing fiberxe2x80x94that the pump fiber is now intact again. Whenxe2x80x94and ifxe2x80x94a pump pulse arrives at the active fiber, the optical power contained in the pulse will be absorbed by the active fiber which, in response to the pulse, simultaneously generates a spontaneous noise called ASE (Amplified Spontaneous Emission), and this ASE signal may then be returned to the pump unit.
The pump unit, which generates the required pump power, may be constructed in different ways. In an expedient embodiment a pump laser is used.
When the pump unit is adapted to detect whether an optical signal is returned from the active fiber in response to the pulsed pump power, it is ensured that the pump unit can switch between the operational state and the safety state in dependence on the returned signal.
When the pump unit is adapted to generate the pulsed pump power as pulses repeated with a given frequency, it is ensured that also the returned ASE noise, in the situation where the pulses arrive at the active fiber, will have this frequency, a corresponding ASE pulse being returned for each emitted pulse. Therefore, the pump unit may expediently be adapted to perform the detection of whether an optical signal is returned from the active fiber in response to the pulsed pump power, by detecting whether an optical signal with the given pulsation frequency is received.
Then, the pump unit may be adapted to remain in the safety state if it is detected that no optical signal is returned from the active fiber in response to the pulsed pump power, and to switch to the operational state if it is detected that such a signal is returned.
Switching from the safety state to the operational state may take place via an intermediate state in which the pump unit can give a continuous pump power superimposed by a pulsed signal. This ensures that in this intermediate state the active fiber may be given a sufficient pump power for it to operate practically normally and therefore to amplify any communications signals, while enabling it to be controlled by means of the pulses whether the connection is still intact until communications signals proper are received. Expediently, as stated in claim 9, the superimposed pulsed signal in the intermediate state may have the same shape as the pulsed pump power in the safety state. As a result, the same detector circuit may be used in the two states.
It will therefore be expedient that optical information signals may moreover be transferred from the active fiber to the pump unit, and that the pump unit comprises means for detecting whether such information signals are received.
A particularly expedient embodiment is obtained when the said optical information signals are transferred from the active fiber to the pump unit via the pump fiber, as the system then just needs one fiber capable of serving as a transmission fiber and pump fiber, and moreover capable, in the safety state, of transferring the pulsed pump power and the possible response to this.
When the pump unit is in the operational state, it may be adapted to remain in this state as long as information signals are received, and to switch to the safety state if no information signals are received.
When the pump unit is in the safety state, it may be adapted to remain in this state if no returned optical signal in response to the pulsed pump power is detected, and to switch to the intermediate state if such a signal is detected.
When the pump unit is in the intermediate state, it may be adapted to switch to the operational state if information signals are received, to switch to the dwell state if no returned optical signal in response to the pulsed pump power is detected, and to remain in the intermediate state if a returned optical signal in response to the pulsed pump power is detected and no information signals are received.
Finally, the pump unit may be adapted to inhibit the detection of whether an optical signal in response to the pulsed pump power is returned from the active fiber, until a selected period of time has elapsed after the transmission of each pulse from the pump unit. This ensures that the detector circuit ignores the reflections that will be returned from the pump fiber, irrespective of whether this is intact or broken, and instead exclusively detects the ASE noise which can only originate from the active fiber, and which will last considerably longer than the reflections from the pump fiber.
As mentioned, the invention also relates to a method of preventing emission of optical power exceeding a prescribed safety limit on interruption of an optical fiber which transfers pump power from a pump unit to an active fiber. When the mean power of the pump power is changed in response to a signal received from the active fiber such that the mean power assumes a value below said safety limit if said signal is not received, and assumes a nominal value if said signal is received, it is ensured that the mean power may automatically be reduced to a safe level when a break occurs on the optical fiber.
This may expediently take place in that the mean power below said safety limit is generated by pulsing the pump power with a given frequency, and that the signal received from the active fiber is detected by detecting whether a signal with the given pulsation frequency is received.